DE 100 60 225 A1 generally discloses a method of this type, wherein both the wheel braking pressure and the volume uptake of the wheel brakes is determined in a braking operation. The determined values are compared with a previously defined evaluation threshold in a p/v diagram, and an alarm signal is triggered and output when the evaluation threshold is exceeded.
A monitoring arrangement of this type is of extreme importance above all for an electrohydraulic vehicle braking system wherein the brake fluid may become contaminated with gases, e.g. air or steam, in the course of time. The result is that the per se incompressible brake fluid becomes compressible. The consequence, in turn, is that considerably more brake fluid must be conducted into the braking system for reaching a defined wheel braking pressure than this would be the case regarding a normal (i.e. uncontaminated) brake fluid with low compressibility. In a braking system where the brake fluid is conducted from a pedal-operated master brake cylinder into the wheel brakes it is possible for the driver to determine himself/herself that the pedal travel is becoming longer and that the “feel” of the brakes becomes inexact when the brake fluid is charged with air or gases. This ‘monitoring’ possibility is lost in an electrohydraulic braking system. This is because with a wheel system of this type the wheel brakes are connected to a high-pressure accumulator by means of valves for build-up of a wheel braking pressure. A pressure in conformity with the driver's request can be built up in the wheel brakes by appropriately actuating the valves. Because an unlimited pressure fluid quantity is principally available in the high-pressure accumulator, which is equipped with pumps in addition, it is possible to build up a sufficiently high braking pressure even with compressible gasses present in the brake fluid without a changing pedal feel. In this case, the driver is unaware of the contamination of the brake fluid with air.
Admittedly, an electrohydraulic braking system of this type generally includes also a master brake cylinder, which is normally separated from the wheel brakes though and becomes connected with said only if the pressure supply by way of the high-pressure accumulator fails. Besides, the pedal-operated master brake cylinder is required to determine the driver's request concerning a defined vehicle deceleration. To this end, among others the force is determined used by the driver to depress the brake pedal.
The situation becomes problematic when the electrohydraulic part of the braking system fails and the wheel brakes become hydraulically connected to the master brake cylinder to build up braking pressure in a conventional fashion by application of the brake pedal. In this case, the volume of the master brake cylinder will no longer be sufficient to build up an appropriately high braking pressure with the compressible brake fluid. Also, it is not possible to increase the size of the master brake cylinder as desired, because its length is limited by the maximum possible pedal travel and its diameter is limited by the pedal force the driver can maximally apply.
Therefore, it is absolutely necessary to monitor an electrohydraulic vehicle braking system of the above-mentioned type with respect to the compressibility of the brake fluid, that means especially with respect to the contamination of the brake fluid with gas or air.
However, the following problem is encountered when realizing a monitoring mechanism: The volume uptake is determined by the elasticities within the vehicle brake and its supply lines. These elasticities are specific for the braking system of a defined vehicle and are subject to defined tolerances under aspects of manufacture. These tolerances are easily accounted for and may therefore be easily taken into account when determining the evaluation threshold. Apart from the elasticities of the vehicle brake, however, there is also a clearance, that means the distance which the actuating members of a braking system (e.g. the brake pistons of a disc brake system) must cover before a braking effect commences. This clearance is constantly changing within a relatively wide range. Among other things, this has two reasons. On the one hand, the resetting members meant to adjust a small clearance after a braking operation, do not act precisely. On the other hand, the actuating members are floatingly supported so that the accelerations of the vehicle cause displacement of the actuating members. Thus, e.g. the actuating pistons of a disc brake system, which are generally arranged transversely to the longitudinal vehicle axle, are pulled away from the brake disc by high lateral acceleration forces. However, vibrations of the vehicle due to rough road conditions can also cause a change of the clearance. The clearance resulting is referred to as a dynamic clearance.
The detectable variations of the clearance are in the same order as the changes appearing due to an air burden. In a method wherein the volume uptake at a defined braking pressure is measured, it is therefore initially impossible to make a distinction whether this volume uptake has been caused by a large clearance or by a major intake of air or gas.
To avoid spurious alarms, the evaluation threshold must be set such that an alarm is prevented even with an accidentally large clearance. However, the result would be that the method is relatively insensitive. It may occur that a great air burden is not detected because based on a small clearance, the evaluation threshold is not reached in spite of a great air burden.